The invention refers generally to a pressure sensor as well as the use thereof in a Fluid Tank which serves for the storage and transport of fluids. The invention refers also to a fluid tank with a pressure sensor.
The invention refers in particular to a pressure sensor and the use thereof in the automotive sector wherein the pressure sensor is in contact with an aggressive fluid in a fluid tank. By fluids is meant in general both liquid and gaseous medium.
A pressure sensor comprises as essential components a housing with a pressure passage to which the medium is applied of which the pressure is to be measured. The known pressure sensors have normally as a pressure passage a pressure port extending into the housing. The pressure passage or if need be, the pressure port is fitted with a micro-mechanical pressure sensing element (sensor chip) arranged in the housing. The pressure sensing element is arranged in a manner that it closes the pressure passage, so that through this a pressure acts on the pressure sensing element.
Such a pressure sensor is, for instance, described in U.S. Pat. No. 7,024,737 B1. Therein, a pressure sensing element is mounted with the interposition of a glass socket with its membrane above the pressure passage of a pressure port, so that an applied and to be measured pressure in the pressure passage of the pressure port acts on the membrane. On the opposite side of the membrane, which mostly and also in the following is designated as the front side of the pressure sensing element, measuring resistors are arranged to identify the applied pressure or a change in pressure by means of the change in resistance. The change in resistance is, as is known, a result of a deflection of the membrane out of its resting position as a consequence of a change of the applied pressure and the associated tension and pressure loading of the membrane. A first processing of the signal and possible temperature compensations take place using an integrated circuit which is integrated on a ceramics board in the pressure sensor.
Depending on the application, the pressure measurement takes place in different measuring routines, for instance for fluid level measurements either hydrostatically from an atmospheric pressure deviating pressure inside a tank, e.g. the fluid tank, alternatively in an open tank, or as differential pressure. By the latter is meant for instance the measurement of the pressure inside a liquid medium, e.g. at the bottom of a fluid tank, relative to the pressure above the fluid level in comparison to a measurement principle of a single sensor for which the pressure measurement takes place absolutely, i.e. against a in the sensor adjusted vacuum or reference volume, or relatively, i.e. in reference to a known absolute pressure in an open or closed measuring cavity of the sensor. A pressure sensor which uses the first mentioned measurement principle is generally designated as absolute pressure sensor, in latter case as differential pressure sensor, wherein the above described pressurized hydrostatic measurement can principally take place through a suitable design of the measuring cavities with only one pressure sensor.
For measuring pressure, the pressure sensor is in contact with the fluid itself or with its vapors. The materials of the pressure sensor therefore have to be resistant against the various elements of the fluid. For the pressure measurement, e.g. in fuel systems with modern fuel, the resistance is not only necessary against petrol, but also against alcohols such as methanol and ethanol as well as against water. These substances occur mainly through adding alcoholic bio-fuel.
The pressure measurement can be required for the purpose of controlling and/or of monitoring functions in various applications. For instance, the pressure measurement takes place in a fuel delivery module which is arranged in the fluid tank as separable component and among others comprises a fuel pump and a filter. Also in the exhaust system for the regulation of urea injections for the reduction of nitrogen emission, the pressure measurement takes place.
Inside a fluid tank, pressure measurements are used for further purposes. In this way, a pressure sensor in a fluid tank can serve, e.g. as level indicator of the fluid tank, or for diagnosing leaks in the fluid tank system, for the control of the tank ventilation devices to diminish under- or excess-pressure in the fluid tank, or equally to control a restraint system for vapor emissions of the fluid. Also in other spatially to the fluid tank connected components, pressure measurements are required.
Such a fluid tank comprises among further components at least a pressure passage which can be closed via a tank flange, and a pressure sensor for the display of the pressure in the fluid tank. By tank flange, a cap of such tank openings is generally understood which besides the filling opening is inserted in the fluid tank to install components such as, for instance, a fuel delivery module, or to enable inspections.
Because of the aggressive medium coming in contact with the sensor, in current embodiments of the fluid tanks, the associated pressure sensors are designed in a way that apart from the rear side of the membrane only the pressure ports come in contact with the medium. In U.S. Pat. No. 5,621,176 A, a similar pressure sensor for measuring the internal pressure of the fluid tank is described. Such pressure sensors as well as such sensors to indicate fluid levels using the above described hydrostatic differential pressure measurement are mounted on the housing wall of the fluid tank, so that only the pressure port extends into the fluid tank. This construction connects the installation of the pressure sensor with the construction of the tank which amongst others requires higher standards in tightness of the installed components, and in particular of the conditioning and calibration of the sensors during an early installation phase.